I am just getting to grips with how my recently purchased 2013 A6 HYBRID works, as it is somewhat complicated. I haven't driven it much in last few weeks as I don't like getting it dirty !  However, if I leave it unused for a week it does tell me the charge on the 12 volt batteries is low - not good for the many 12 volt systems which will show up fault codes. So I took the cover off the batteries in the boot, did some checks with Varta/ Exide and found that my 2 x 12volt batteries are the original Factory fitted items dated 2012 - so when the car was manufactured. Amazing that they have lasted 14 years ! I have purchased new VARTA replacements, but have not fitted them yet. Although, I have an Official Audi workshop manual procedure for this, it is not 100% clear whether in this instance it is necessary for the high voltage system to be de-activated ( which I would definitely not get involved with if needed - as it is a safety issue ). Does anyone know if this is required. Both batteries have to be changed at same time, disconnecting a main earth cable first, and then a scanner has to reset all the low voltage codes after completion. Anyone with knowledge ?       

I guess there are too few around to expect a reply. But I have done it now - changed the batteries and reset the codes relating to low voltage on the 12 volt systems. The original batteries were 75ah Varta and 36ah Exide. The modern equivalents from Varta are 80ah and 44 ah ( same dimensions and type but improved performance ). The 75ah one was struggling to keep a good charge, but the 36ah only measured 1.3 -1.5 volts when I removed it, so well overdue for replacement. I was lucky it didn't cause any other problems.

Thanks for the information.